Today's mobile and desktop systems implement sleeping states as defined by ACPI (Advanced Configuration and Power Interface), whereby the platform and operating system are placed in a sleeping state when there is currently no work to be done. Entry into these low power states is determined through operating system (OS) specific actions and may include (but not be limited to) end user interaction (e.g., lid closure, power button press, etc.) or system inactivity for some period of time. The original concept of Intel's Instantly Available PC or Microsoft's OnNow initiative is that the PC would rapidly wake from the sleeping state and be useable in very short timeframe, much like what occurs with appliances such as VCRs and microwave ovens. In practice however, this has not been the case with PCs, which have historically taken 6–10 seconds to resume normal operation from a sleeping state.
Most PCs implement one or more sleeping states, known as Sx states. S1 places the system in a low power state, but does not typically power off any devices. Namely, the processor is still powered, as are most devices. Historically, this type of sleeping state is less desirable from a power point of view, but has been able to achieve faster wake times. States lower in power than S1, namely S2 and S3, power off a large portion of the system, including the processor. For these states, it is necessary for the system BIOS to initialize the processor and memory as well as some number of system level devices, prior to handing control back to the operating system. This BIOS initialization time after an S2 or S3, is known as the BIOS resume time, and it is a critical component in achieving shorter resume times for the PC.
There are two parts of the problem for achieving faster wake times. The system BIOS wake time occurs first, after the end user has requested that the machine wake from the sleeping state until the time at which the BIOS passes control back to the operating system. The second part of the problem is the OS resume time. This is defined as the time it takes for the OS to reinitialize device drivers and bring the machine back to a useable state, where the end user can begin interacting with the machine. Originally, the OS wake time was very long for initial ACPI implementations, on the order of 4–6 seconds. This has been dramatically reduced on current operating systems, and some systems have achieved <1 s OS wake time. As a result, the BIOS resume time comprises the bulk of the time lapse between activation of a wakening event and the PC becoming available for use.